(Bill Gallo Jr.) An “unusual event” was declared at the Salem 1 nuclear reactor here this morning after the plant shut down and fire alarms sounded, a spokesman for the plant’s operator said.

An unusual event is the least serious of four emergency classifications at a nuclear power plant.

Salem 1 automatically shut down at 10:15 a.m., according to Joe Delmar, spokesman for the reactor’s operator, PSEG Nuclear.

According to Delmar what caused the reactor to trip remains under investigation, but testing was occurring on the reactor’s emergency cooling system at the time.

After the reactor shutdown, multiple fire alarms sounded, according to Delmar.

Per federal regulations, PSEG Nuclear is required to investigate all fire alarms within 15 minutes.

Since they were unable to do so, an unusual event was declared. Once it was confirmed there was no fire at the plant, the unusual event was declared over 12:49 p.m.

When Salem 1 will return to service is unclear, Delmar said early this afternoon.

The reactor is stable, Delmar said, and there was no release of radiation during the event.

Officials from the federal Nuclear Regulatory Commission, which oversees the operation of the nation’s 104 nuclear power plants, was notified of the unusual event declaration. Local, county and state emergency management officials were notified of the unusual event declaration.

The last time an unusual event was declared at the Island generating complex here in Lower Alloways Creek was in August 2011 after the East Coast earthquake rattled th region.

Salem 1 is one of three nuclear reactors operated by PSEG Nuclear at its Artificial Island generating complex here along the Delaware River.

Salem 2 was not impacted and remains operating at full power. The third reactor at the site, Hope Creek, is currently shut down for a scheduled refueling outage.

The three reactors comprise the second largest nuclear generating complex in the United States.

When all three reactors are operating at full power they can produce enough electricity to power three million homes.

PSEG Nuclear is a subsidiary of Newark-based Public Service Enterprise Group.